code: def test_all_None(df):
      """
      Test output where value is None.
      """
      result = df.encode_categorical(names=None)
  
      expected = df.astype({"names": "category"})
      assert expected["names"].equals(result["names"])
  
imports: import datetime
  import numpy
  import pandas
  import pytest
  from hypothesis import given, settings
  from pandas.testing import assert_frame_equal
  from janitor.testing_utils.strategies import categoricaldf_strategy, df_strategy
label_desc: Use image node_id 20 for calling a global function with example usage: test_all_None(df) without return types